In a manufacturing process of a ferromagnetic metal, in particular, thin steel sheets, foreign matter adhering to or caught up in a roll installed in a manufacturing line sometimes produces concavo-convex shape on the roll itself, and this concavo-convex shape may be transferred to resulting steel sheets as roll-generated flaws.
Such roll-generated defects include concavo-convex shape surface defects that each exist on a rough steel sheet surface (Ra=0.5 to 2 μm) and have a smooth outline (radius of curvature R≧10 mm), a concavo-convex level of 5 μm or lower, and an area of 10 mm2 or larger. Such defects are hereinafter referred to as fine concavo-convex shape surface defects. FIG. 4 is a schematic diagram of a cross-section of a fine concavo-convex shape surface defect. The size of each fine concavo-convex shape surface defect is approximately in the range of 10 to 1000 mm2 in area, but is 5 μm or lower in concavo-convex level as described above. The lowest concavo-convex level is approximately 1 μm, which is very small and is of the same order as the surface roughness.
Most of roll-generated defects have a relatively high concavo-convex level and are visible, and thus they can be easily found in the course of a manufacturing line. However, such fine concavo-convex shape surface defects have concavo-convex levels as small as the surface roughness of steel sheets. The optical difference between these fine defects and surface roughness is small, and thus the defects cannot be easily found in the course of a manufacturing line when observed without any treatment. However, once the surface is coated and the surface roughness is filled with a coating agent to make a smooth surface, the fine defects become clearly visible thereby posing serious problems in terms of appearance. Therefore, detection of fine concavo-convex shape surface defects prior to coating is an important issue in quality control.
The shape of fine concavo-convex shape surface defects includes point flaws like roll-generated flaws described earlier and flaws extending in the longitudinal direction of a steel sheet, such as linear marks or buckling marks.
Such fine concavo-convex shape surface defects are formed by transfer of concavo-convex shape of a roll to a steel sheet, and this situation persists until the roll is replaced or the process is improved. Therefore, early detection and countermeasures are very important also for yield improvement.
To find such fine concavo-convex shape surface defects, threading of steel sheets is stopped during operations in each inspection line in an iron-making process, and thereafter inspectors rasp all the coils with a whetstone and perform visual inspections thereof. After being rasped with a whetstone, convex portions, which are more strongly rasped than concave portions, have a higher reflectance than concave portions. This makes the difference between convex and concave portions clearer and more visible in visual inspections. This method is called whetstone inspection.
Unfortunately, this method necessitates stopping of inspection lines and requires a substantial period of time, thereby reducing the efficiency of operations. To address this situation, a method for automatically inspecting concavo-convex shape defects each having a concavo-convex level on the order of a few micrometers and a smooth outline has been developed. Examples of such an automatic surface analyzer may include the techniques disclosed in Japanese Unexamined Patent Application Publication Nos. S58-86408, H5-256630, H6-58743 and 2000-298102.
However, the technique disclosed in Japanese Unexamined Patent Application Publication No. S58-86408 is a technique for inspecting a mirror surface, and thus cannot be applied to an object with a large surface roughness because of the problem that beams converging into or diffused from concaves and convexes of flaws are masked by beams diffused by surface roughness and thus the flaws cannot be detected.
Also, the technique disclosed in Japanese Unexamined Patent Application Publication No. H5-256630 is intended for steel sheets, but still cannot be applied to objects other than materials having a mirror surface such as stainless steel sheets. Furthermore, this technique is admittedly effective in detecting concavo-convex defects extending perpendicular to the illumination detection, but is insufficient in terms of detection sensitivity for those extending parallel to the illumination detection.
Additionally, the technique disclosed in Japanese Unexamined Patent Application Publication No. H6-58743 is intended for unpolished wafers having rough surfaces, but still has the problem of low detection accuracy because it determines whether a flaw is present on the basis of the total light intensity and thus cannot detect specific signals of individual flaws.
The technique disclosed in Japanese Unexamined Patent Application Publication No. 2000-298102 was developed to address this situation and, accordingly, an apparatus for this technique has very high detection sensitivity. However, this technique requires an angle of incidence as large as close to 90°, thereby making it difficult to introduce a necessary apparatus into an actual operation line. It has also the problem that adjustment of optics is difficult.
In view of general defect-detecting methods without limiting the target thereof to fine concavo-convex shape surface defects, there are detection methods wherein magnetic flux is applied to a sample, for example, the technique disclosed in Japanese Unexamined Patent Application Publication No. H8-160006 for detecting an inclusion existing inside an object using a magnetic flux leakage inspection method. This patent document describes that surface defects can be detected also by using this magnetic flux leakage inspection method in the form of leakage flux signals generated by changes in the surface shape (signals generated by variation or disturbance of magnetic flux due to changes in the shape).
However, the intensity of signals for this magnetic flux leakage inspection corresponds to the shape change level of surface defects (concavo-convex change level), and thus the lowest shape change level of surface defects (concavo-convex change level) detectable in an automatic inspection is limited. Those skilled in the art consider this lower limit to be approximately 100 μm (Magnetic Flux Leakage Inspection Method for Iron and Steel Products, Feb. 28, 2001, NDI Department (Technology and Technique Transfer Committee), Working Group for Quality Control, the Iron and Steel Institute of Japan). In other words, it is recognized that stable detection of surface defects having a shape change level of approximately 100 μm or lower is difficult and signals generated by changes in the shape of approximately 100 μm or lower serve only as noise sources observed in flaw detection for other purposes (e.g., detection of inclusions).
Although such a simple comparison may be inapplicable, signals for detecting inclusions similarly correspond to the size of defects, and thus the lowest detectable size of a defect in the thickness direction of the test steel sheet is still limited. Those skilled in the art consider this lower limit to be approximately 10 μm (Development of a Minute Inclusion Detector for Thin Steel Strips, CAMP-ISU, Vol. 10 (1997)-289, Development of Gouge Defect Detector, Chiba 2CGL, 131st Control Technology Working Group Meeting, Manufacturing Technology Department, the Iron and Steel Institute of Japan, Development of Non-metallic Inclusion On-line Detection System, CAMP-ISIJ, Vol. 7 (1994)-1270 and State-of-the-art Techniques for Internal Quality Measurements in Steel Sheets Production Processes, Kawasaki Steel Giho, 31 1999 4.211-215).
Furthermore, in such magnetic flux leakage inspection, an increase in the sheet thickness would result in a widened range of flaw detection in the thickness direction, thereby resulting in increased background noise generated by the steel sheet, increased magnetizing force being needed for magnetization of the steel sheet, reduced flatness of the surface of the steel sheet leading to difficulty in scanning the steel sheet with a sensor, and other problems. For these reasons, the thicker the test sheet is, the more difficult flaw detection is. With the inclusion detector described in the publications listed above, the lowest detectable size of an inclusion in a steel sheet discussed, whose maximum thickness is 2.3 mm, would probably be much larger than 10 μm (size in the direction of the steel sheet thickness) if they can be detected at all, considering the fact that this inclusion detector is intended for thinner steel sheets used for cans.
Meanwhile, Report of Explorative Research on Seed Technology for Detection of Whetstone-inspection-level Flaws, July 1995, WG for research on cold rolling in rolling, refining, and temper systems, Iron and Steel Technology Committee, the Japan Iron and Steel Federation discloses the results of explorative research on the detection of whetstone-inspection-level flaws conducted by WG for research on cold rolling in rolling, refining, and temper systems, Iron and Steel Technology Committee, the Japan Iron and Steel Federation from September 1994 to July 1995 (Report of Explorative Research on Seed Technology for Detection of Whetstone-inspection-level Flaws). Previous approaches to automatic inspections of fine concavo-convex shape surface defects were mainly based on a light-detecting method, and, to date, there have been no approaches using a magnetic flux leakage inspection method.
Therefore, it can be said that the use of magnetic flux leakage inspection for detecting fine concavo-convex shape surface defects that can be visible only after the surface of a test steel sheet is rasped with a whetstone would not normally be considered by those skilled in the art of magnetic flux leakage inspection.
It could be helpful to provide a practical method for stable detection of concavo-convex shape surface defects that exist on a test object having a surface roughness Ra approximately in the range of 0.5 to 2 μm, wherein the concavo-convex shape surface defects are usually almost invisible and detectable only by whetstone inspection, are difficult to detect by automatic detection, and each have a concavo-convex level on the order of a few micrometers, and also provides an apparatus for the same objective.